The present invention relates to a process of fabricating a resin package used with electronic components, etc.
A resin package such as that of the chip-on-board structure wherein electronic components such as semiconductor chips are mounted on a resin substrate in a flip chip manner and the electronic components are then covered and sealed up with a resin layer such as a silicone resin layer, and a resin package of the pin grid array (PGA) or ball grid array (BGA) structure wherein multiple resin substrates are laminated and bonded together enables outer configurations, through-holes, and chip-mounting recesses to be formed by machining with drills, and rooters in an economical manner yet with high accuracy, because the resin substrates are rich in machinabililty. Instead of ceramic packages, these resin packages are now widely used as packages for ICs, sensors, surface acoustic wave devices, etc.
Basically, it is desired that resin packages can meet requirements inclusive of 1) miniaturization, 2) high accuracy compatible with high-density packaging, 3) compatibility with diverse configurations, and 4) high reliability. Especially for the achievement of ever higher-density packaging, there is now a growing demand for further miniaturization of resin packages. Thus, it is required to arrive at a tradeoff between high-precision micro-machining and high reliability.
Some prior art processes are explained with reference to FIGS. 7, and 8. FIG. 7 illustrates a process of fabricating a resin package using a resin substrate comprising a first resin substrate member on which chips are to be mounted, and a second resin substrate member that is to form a frame member. The second resin substrate 51 of about 0.5 mm in thickness is provided therein with rectangular windows 52a, through-holes 52b, etc. by means of machining with a drill or rooter. Then, the second resin substrate 51 is aligned with and placed on the first resin substrate 54 with a prepreg 53 of about 0.1 mm in thickness sandwiched between them (see FIG. 7(A)). The assembly is heated to about 200.degree. C. under pressure to laminate and bond together the resin substrates while the prepreg remains fluidized (see FIG. 7(B)). In this case, the first resin substrate 54, too, has been drilled or otherwise machined. Following this, the thus laminated and bonded assembly is separated into sections as by dicing, as shown by broken lines (see FIG. 7(C)). After a chip is mounted in the rectangular window 52a, the window 52a is sealed up by a lid (not shown). If required, the first resin substrate 54 and/or the second resin substrate 51 are provided with conductor patterns. So far, resin packages with chips incorporated therein have been formed in this manner.
FIG. 8 illustrates a typical prior art resin package fabrication process sequence wherein resin substrates are laminated and bonded together. At the beginning, a conventional resin substrate formed of thermosetting resin such as BT resin has a relatively low glass transition temperature (Tg:Tg=T.sub.1.degree. C.), and generally possesses hardness and elasticity capable of machining because of including a high-molecular or polymer component having an unsaturated degree of polymerization. The resin substrate is cut, drilled or otherwise machined in this state. Such resin substrates upon machining are placed one upon another with a prepreg sandwiched between them, or a sealing agent, an adhesive agent or the like is potted on the resin substrates. Then, the resin substrates are heated to a temperature Tb (.degree. C.) that is higher than the glass transition temperature of the aforesaid wiring board to laminate and bond or fix together them. After this, the bonded laminate is spontaneously cooled and diced into discrete sections. So far, the resin package has been fabricated in this way.
Since the bonding of laminated resin substrates such as BT substrates by heating is carried out at a temperaure that is greater than the glass transition temperature thereof, however, the prior art processes have such contradictory problems as explained below.
(1) A BT substrate that is one example of the resin substrate is easily machinable if the glass transition temperature (Tg) thereof is between 170.degree. C. and 190.degree. C. However, the BT resin, if it has a glass transition temperature (Tg) brought up to 200.degree. C. or higher, becomes hard due to its increased degree of polymerization, with its machinability becoming worse. This BT resin then provides a resin substrate that is brittle and susceptible to cracking, chipping or peeling upon machining, and so cannot stand up well to high-accuracy machining. At the same time, tools such as blades of drills or rooters wear away excessively, resulting in a drastic decrease in the number of the holes to be formed.
(2) When the glass transition temperature (Tg) that resin substrates have from the beginning is lowered, on the other hand, the temperature to which the resin substrates are heated at post-steps for their lamination and bonding is much higher than the initial glass transition temperature (Tg). Consequently, there is an increase in the degree of polymerization of the resin substrates at the heating temperature applied at the post-steps. This increase is then associated with pressurization during heating to give rise to thermal deformation of the resin substrates such as warpage, and shrinkage, as shown in FIG. 7(C) and the process sequence of FIG. 8. As a result, difficulty is involved in dicing the laminated assembly into discrete resin packages. Moreover, a problem arises when chip components are mounted in a resin package cavity in a flip chip bonding manner, as can be seen from FIG. 9 that is a sectional schematic showing deformation of a prior art resin package. That is, the heights of bumps between the chip components vary and, hence, a portion of insufficient bonding strength occurs. This portion may in turn cause failures such as bump displacements, resulting in a lowering of the reliability of resin package products. To make electrical connections between a largely deformed package and chips, means having a large degree of dimensional freedom, e.g., wire bonding must be selected, and package size increases, accordingly. In other words, miniaturization is not achievable.
(3) If a prepreg is heated at a temperature that is lower than the glass transition temperature (Tg), its thermal deformation at post-steps may then be avoided. However, insufficient bonding strength makes it impossible to bring resin substrates in close contact with each other, resulting in a lowering of sealing performance and, hence, a lowering of the airtight reliability of resin packages. When a resin package is fabricated by laminating and bonding together resin substrates with a prepreg sandwiched between them, it is required to heat the prepreg to at least 200.degree. C. exceeding the aforesaid glass transition temperature (Tg), thereby enhancing the fluidity of the prepreg to prevent voids from remaining therein, and increasing the bonding strength of the prepreg with respect to the resin substrates.
Generally speaking, the aforesaid prior art processes fails to gain the purpose of providing a resin package that can be micromachined with high precision, and is excellent in reliability as well.
In this connection, for instance, JP-A 8-316374 shows a BGA package of the structure wherein one side thereof is molded with a sealing material composed mainly of resin, and discloses that the sealing molding with a sealing resin composition is carried out at a temperature that is lower than the glass transition temperature (Tg) of the cured resin composition. The publication alleges that the amount of warpage of the BGA package can be reduced. To take full advantage of such effect, however, the type of resin in the resin composition, and the type of curing agent used should be properly selected. Thus, not only are there many material constraints but also the resultant product becomes poor in heat-resistant reliability because the selected resin has a low curing temperature.
One object of the invention is to provide a solution to the aforesaid problems by the provision of a process of fabricating a resin package of excellent reliability while taking advantage of the excellent machinability that a resin substrate possesses. Another object of the invention is to provide a resin package fabrication process having high efficiency in addition to the advantages as mentioned just above. Yet another object of the invention is to provide a resin package fabrication process that enables a package to be miniaturized.